Optical devices for the measurement of acceleration or displacement are known in the art. Such devices have utility in a number of different industrial applications, and specifically have utility in oil/gas applications such as seismology and well-deviation monitoring.
Typically, optical accelerometers or displacement devices operate through a connection of an optical element to a mass usually positioned inside of a housing. As a force acts on the mass, the mass moves within the housing, thereby imparting a stress to the optical element indicative of the force, be it a constant force like gravity, or a varying (dynamic) force as might be experienced in seismic detection. The optical element in such devices is typically an optical fiber, perhaps containing a fiber Bragg grating (FBG). A FBG, as is known, is a periodic or aperiodic variation in the effective refractive index of an optical waveguide, similar to that described in U.S. Pat. Nos. 4,725,110 and 4,807,950 entitled “Method For Impressing Gratings Within Fiber Optics,” to Glenn et al. and U.S. Pat. No. 5,388,173, entitled “Method And Apparatus For Forming Aperiodic Gratings In Optical Fibers,” to Glenn, which are incorporated by reference in their entireties. As the FBG is stressed by the force, the Bragg reflection wavelength of the FBG shifts accordingly, which may be interrogated to quantify the detected force. An example of such a device is disclosed in U.S. Pat. No. 6,175,108, which is incorporated herein by reference.
Optical fiber accelerometers or displacement devices can also be interrogated by interferometric means. For example, in U.S. patent application Ser. No. 09/410,634, filed Oct. 1, 1999, and Ser. No. 10/068,266, filed Feb. 6, 2002, both of which are incorporated herein by reference, a coil of optical fiber is coupled to or around the mass. The length of this coil is bounded by FBGs, which essentially act as reflectors. By interferometrically assessing reflections from these FBGs, the length of the coil can be determined, which is indicative of the force experienced by the mass.
While these prior art approaches function well to measure acceleration (dynamic forces) or displacement (constant forces), they generally require that the optical element at issue (i.e., the FBG or coil) be pretensioned, as is it not desirable for the optical element to ever become “slack” against the mass during operation. Tensioning of the optical element can lead to shortened lifetimes of the device and raises general reliability concerns in some applications. Additionally, while interferometric interrogation is highly accurate to determine changes of length in optical waveguides, it also requires more extensive optical interrogation systems than does mere assessment of a Bragg wavelength shift from an FBG.
It is known that optical sensors are sensitive to temperature. For example, in an FBG based optical sensor, the FBG will expand or contract in response to increases or decreases in temperature in accordance with the coefficient of thermal expansion (CTE) of the (usually) quartz FBG element. Additionally, the index of refraction of the FBG (or other waveguide) will change with temperature. Changes in temperature will cause the spacing, Λ, of the grating in the FBG to expand or contract, and will also affect the index of refraction, both of which affects the Bragg reflection wavelength, λB, of the sensor. (As is known and as is explained in the incorporated references, λB ∝ 2neffΛ, where neff is the index of refraction of the core of the waveguide). These temperature-induced Bragg reflection wavelength shifts are preferably compensated for when measuring acceleration or displacement.
Accordingly, there is room for improvement in the art of optical accelerometers and/or displacement devices, and this disclosure provides an alternative approach to the prior art having significant advantages.